Symmetrical quasi-absorptive RF bandpass filters

A type of symmetrical RF bandpass filters (BPFs) with quasi-absorptive functionality are presented. They are composed of a bandpass section that is loaded at its input/output ports with nearly complementary resistively terminated bandstop sections. In this manner, the input-signal energy that is not transmitted by the bandpass section is dissipated by the loading resistors of the bandstop sections to attain the quasi-reflectionless behavior. The theoretical analysis of the first-order symmetrical quasi-absorptive BPF stage is detailed. In addition, synthesis examples of higher attenuation and sharper rejection designs based on in-series-cascade arrangements, higher order realizations, and cross-coupling techniques are illustrated. A coupling-matrix-level loss analysis of the devised symmetrical quasi-reflectionless BPF configuration for uniform and nonuniform quality-factor (Q) distribution is also carried out. Furthermore, for experimental validation, 2.5-GHz microstrip BPF demonstrators consisting of an in-band linear-phase two-stage in-series-cascade circuit and a second-order prototype are manufactured and characterized.